1. Field of the Invention
The invention is directed to a drive unit, particularly in a motor vehicle, with an internal combustion engine that is charged in two stages by two exhaust turbochargers and which has a charge air supply having a low-pressure compressor, a high-pressure compressor, an intermediate charge air cooler arranged between the low-pressure compressor and the high-pressure compressor, and a main charge air cooler downstream of the high-pressure compressor. The drive unit has an exhaust gas discharge including a high-pressure turbine, a low-pressure turbine and an exhaust gas aftertreatment device downstream in the exhaust train, and a cooling installation with a cooling circuit and a heat recovery circuit, which is hydraulically decoupled from the latter.
2. Description of the Related Art
A drive unit is disclosed in DE 10 2006 010 247 A1. A feature of this known drive unit is that its first cooling circuit and its second cooling circuit, which functions as a heat recovery circuit, are in thermal contact via at least one heat transfer device. For example, an EGR cooler and an exhaust gas heat exchanger arranged downstream in the exhaust train of an exhaust gas aftertreatment device are provided as heat transfer devices and are arranged in this sequence successively, i.e., in series one behind the other, in the second cooling circuit and, therefore, transfer thermally coupled heat to a work medium flowing through the second cooling circuit. The work medium of the second cooling circuit is heated as it flows through the EGR heat exchanger and then through the exhaust gas heat exchanger, changed at least partially into its vaporous aggregate state, and superheated. The work medium is then used in an expander to produce useful power. The work medium then flows in the second cooling circuit through at least one air-cooled condenser and then, again in liquefied aggregate state, through the main charge air cooler and intermediate charge air cooler and possibly through additional heat exchangers. Accordingly, in this known system, both the charge air and the engine cooling water are used in addition to the exhaust gas as heat sources which are connected in series, one behind the other, and receive the heat of the respective work media.
Further studies have shown that the expander only starts to work efficiently at high pressures. However, with a serial flow through the above-mentioned heat exchangers the temperatures of the charge air, engine coolant and exhaust gas are not sufficient to evaporate the work medium at high pressures. It must be considered in this respect that the charge air which is compressed in two stages and the internal combustion engine must both be sufficiently cooled in all performance ranges to ensure an advantageous operation of the combustion engine and to prevent its overheating. Therefore, large quantities of work medium must be pumped through the second cooling circuit in the known system, with the result that this amount of work medium cannot be completely vaporized even by the heat of higher-output heat sources when flowing through the latter so that a separation system for the liquid phase is mandatory. Further, because of these factors, the returned exhaust gas cannot be cooled to the desired low temperature level when flowing through the one or more EGR heat exchangers because the work medium is already preheated when entering the one or more EGR heat exchangers.